Protective system for thermal transducer

ABSTRACT

THE HEATER OF A THERMAL TRANSDUCER IS CONNECTED IN SERIES WITH A CIRCUIT BREAKER CONTROLLED BY A PROTECTIVE RELAY WHICH OPERATES, UPON THE TEMPERATURE OF THE HEATER REACHING A PREDETERMINED LIMIT, IN RESPONSE TO THERMAL RADIATION FROM THE HEATER IMPINGING UPON A PHOTOCONDUCTOR OR TO ELECTRON   EMISSION FROM A CATHODE FORMING PART OF OR CLOSELY JUXTAPOSED WITH THE HEATER.

United States Patent [72] Inventors Karl-Heinz Heidenreich Neuhausen'Erms; Friedrich Neuscheler, Walddorf ub Tubingen, Germany [21] Appl. No. 756,878 [22] Filed Sept. 3, 1968 [45] Patented June 28, 1971 [73] Assignee Wandel & Golte'rmann Reutlingen, Germany [32] Priority Aug. 31, 1967 l [33] Germany [54] PROTECTIVE SYSTEM FOR THERMAL TRANSDUCER 7 Claims, 3 Drawing Figs. 52 use! 317/41, 219/505 51 1m.c| H02h9/02 [50] Field ofSearch 317/41,40, I 132;2l 9/487,501,505;73/359,362 (R) [56] References Cited UNITED STATES PATENTS 2,861,239 11/1958 Gilbert 73/359 2,929,968 3/1960 Henisch 317/41 3,186,105 6/1965 Nye 317/132 3,236,451 2/1966 Josephs 219/501 3,361,941 1/1968 Fickweiler 219/505 Primary Examiner-Harold Broome Att0riteyKarl F. Ross ABSTRACT: The heater of a thermal transducer is connected in series with a circuit breaker controlled by a protective relay which operates, upon the temperature of the heater reaching a predetermined limit, in response to thermal radiation from the heater impinging upon a photoconductor or to electron emission from a cathode forming part of or closely juxtaposed with the heater.

PATENTED JUN28 197:

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20' ELECTRON- I EmsswE Karl-Heinz Heidenreich Friedrich Neusch eler IN VliN'l'HRS.

{K rl 12D Attorney PROTECTIVE SYSTEM FOR THERMAL TRANSDUCER Our present invention relates to a thermal transducer having an electrically energized heating element juxtaposed with a thermocouple, the latter being connected to an indicator or other load. Such transducers can he used, for example, to integratc a train of pulses or to rectify a low frequency alternating signal.

Since the heaters of such transducers are rather sensitive to overloading,-it is customary to connect them in parallel with a protective circuit which may include breakdown diodes or thermistors designed to shunt out excessive'signal currents. While such protective circuits operategenerally satisfactorily in the presence of sinusoidal input voltages, they have a tendency'to short circuit the heater in response to rectangular pulse trains whose integrated value corresponds to an effective current flow well within the range of tolerance of the heater.

it is. therefore, the general object of our present invention to provide a protective circuit arrangement for such thermal transducers which avoids this drawback and responds only to actual overload conditions.

This object is realized, persuant to our present invention, by the provision of a self-locking circuit breaker in series with the heater, the circuit breaker being actuated by a monitoring element responsive to thermally induced radiation and juxtaposed with the heater to receive such radiation from it, the monitoring element having a threshold of operation corresponding to a predetermined maximum temperature of the heater.

The thermally induced radiation referred to above may include infrared and/or visible light from the heater thereof or electrons (i.e.beta rays) emitted by the suitably coated heater surface or by a separate cathode disposed in heat-transfer relationship therewith. la the first case, the monitoring element may comprise a photoconductor positioned for irradiation by the heater, preferably in the vicinity of its junction with the bimetal of the thermocouple; in the second instance, the heater and/or its cathode are enclosed in an evacuated envelope also containing an anode which serves as the monitoring element.

The invention will be described in greater detail with reference to the accompanying drawing in which:

FIG. 1 is a circuit diagram of a first embodiment;

FIG. 2 is a circuit diagram of a second embodiment; and

FIG. 3 is a somewhat diagrammatic perspective view of a structure containing some of the elements of FIG. 2.

ln FlG. l we have shown a thermal transducer, generally designated l0, which comprises a heater 34 in the form of an electrical resistance element and a thermocouple 35 forming with that heater a junction 36. The thermocouple 35 works into an amplifier 12 which energizes a load here shown as a voltmeter 13. A terminal l4 of heater 34 receives an input signals, e.g. a train of pulses, whose mean value is to be measured by the meter l3.

The other end of the heater is grounded through an armature 16a and a back contact ofa relay 16 also having a holding armature 16b.

A photoconductive element 17, preferably a semiconductor connected as a diode or a triode, is positioned to receive thermal (e.g. infrared) radiation from the region of heater 34 in the vicinity of its junction 36. This element 17 lies in series with a resistor 24 between ground and a positive terminal 26 so as to constitute therewith a voltage divider controlling the energization of an amplifier 18, here shown as an NPN transistor, whose output circuit includes the winding of relay 16. A biasing potential for the transistor base is derived from a potentiometer 27in parallel with circuit i734 Holding armature I6!) is in series with a manually or otherwise operable. normally closed switch l9. As iongns the signal s does not exceed the limit selected with the aid of potentlomcter 27. transistor l8 remains cut off and relay 16 t unopersted. The thermal transducer 10 then functions in the normal manner. If. however, overload occurs. the intensified radiation impinging upon photoconductor l7 lowers the resistance thereof sufficiently to drive the base/emitter voltage above the threshold of conductivity whereupon relay [6 is energized and locks over its armature 16!; while breaking the circuit of heating resistor 34 at armature 16a. After the overload has subsided. a momentary opening of switch 19 restores the system to its operative state.

FIG. 2 shows a generally similar arrangement wherein, however. the input circuit of an amplifier 25 (which may also include a transistor, as in the preceding embodiment) comprises a vacuum diode in series with resistor 24, this vacuum diode being constituted by an electron-cmissive cathode 20 and an anode or plate 37 within an evacuated envelope 30 surround ing the heater 34. Cathode 20 is positioned for direct heating by resistance element 34 in the vicinity of its junction 36 with the bimetallic element'35. As in the system of FIG. 1, relay l6 responds to the attainment of a predetermined temperature level by the heater 34; in this case, however, the relay is energized by electron emission from cathode 20 at a rate sufficient to trigger the amplifier 25 into conduction.

F l6. 3 shows a physical realization of the transducer of F K]. 2 with its evacuated glass envelope 30 containing the heater 34, the thermocouple 35 and the anode 37, the latter being formed as the tip of a metallic sheet 33 of generally trapezoidal configuration folded along two horizontal lines. Conductor pins 31, passing in the conventional airtight manner through a pressed-glass base 32 of envelope 30. support the anode sheet 33. the heater 34 and the bimetal 35 while connecting these elements in circuit; one of these pins also carries a blob 39 of getter material.

in H0. 3, the cathode 20 of FIG. 2 has been replaced by an electron-emissive coating 20' applied directly to the heating wire 34 in the region of junction 36.

It will be apparent that the several embodiments described and illustrated may be modified in various ways without dcparting from the spirit and scope of our invention as defined in the appended claims.

We claim:

I. A circuit arrangement for integrating discontinuous electric currents, comprising:

thermoelectric transducer means including a resistor connectable across a source of current to be integrated and a thermocouple closely juxtaposed with said resistor for generating an output voltage dependent upon the heating ofsaid resistor by such current;

a load connected to said thermocouple for energization by said output voltage;

temperature-monitoring means independent of said thermocouple disposed close to said resistor for detecting a predetermined temperature limit; and

protective circuitry for said resistor including switch means responsive to said temperature-monitoring means for effectively disconnecting said resistor from said source upon attainment of said temperature limit, thereby preventing overloading ofsaid transducer means.

2. The circuit arrangement defined in claim 1 wherein said switch means comprises a self-locking relay.

3. The circuit arrangement defined in claim 1 wherein said temperature-monitoring means comprises an element responsive to thermally induced radiation from said resistor.

4. The circuit arrangement defined in claim 3 wherein said element is a photoconductor.

5. The circuit arrangement defined in claim 3 wherein said element is an anode, said temperature-monitoring means further including an electron-emissive cathode subjected to the temperature of said resistor and an evacuated envelope surrounding said anode and cathode.

6. The circuit arrangement defined in claim 5 wherein said cathode is a coating on said resistor.

7. The circuit arrangement defined in claim 6 wherein said resistor is it wire and said thermocouple is a bimetal physically joined to sttid wire. said anode being disposed in the region of the junction between said bimetal and mid wire. 

